Conventional advanced session control protocols, such as a session initiation protocol (SIP) make use of so-called “signaling” messages, which are messages enabling a terminal to request a connection with another terminal, and also messages signaling that a telephone line is busy, or that the called telephone is ringing, or indeed that such a telephone is connected to the network and may be reached in such and such a manner.
The SIP was defined by the Internet engineering task force (IETF) in a document RFC 3261. This protocol makes it possible to set up, to modify, and to terminate multimedia sessions in a network using IP. SIP has subsequently been extended, in particular in document RFC 3265. This extension makes event notification procedures possible.
SIP is used in particular in infrastructures of the IP multimedia subsystem (IMS) type. The IMS was defined by the 3rd Generation Partnership Project (3GPP) Standardization Organization and by Telecommunications and Internet Converged Services and Protocols for Advanced Networking (TISPAN). It comprises a network architecture that was introduced by 3GPP for mobile networks and then extended by TISPAN for fixed networks. This architecture enables multimedia sessions to be set up dynamically and controlled between two clients, and it also enables resources to be reserved in the network used for transporting multimedia streams. By means of this architecture, network operators can conveniently implement a management policy, can provide a predetermined quality of service (QoS), and can calculate how much to bill clients. At present, IMS makes it possible to access services such as telephony, video telephony, and instant messaging, and it also manages interactions between them. It should be recalled for example that instant messaging (IM), also referred to as “chat”, enables text messages and files to be exchanged almost instantaneously, thereby enabling two users connected to the same network (such as the Internet) to have an interactive dialog.
When a user seeks to benefit from services made available by an IMS network, the user sends signaling messages to the network that may include in particular various types of request.
Firstly, apart from exceptions (such as certain emergency calls), the user's client device must register with the network. When the network is incapable of establishing a link between the registration and an earlier registration (e.g. as a result of a network failure, or as a result of the terminal being switched off for a duration longer than a predetermined value), the registration is considered as being an initial registration. After an initial registration, the user's client device must periodically send requests to the network in order to confirm that it desires to maintain its registration.
Thus, in order to be able to register client devices, IMS networks comprise one or more registration servers known as “serving-call server control function” (S-CSCF) servers suitable (among other functions) for managing the procedure for registering devices connected to the network.
In addition, such networks comprise one or more servers known as “interrogating-call server control function” (I-CSCF) servers, which indeed are often physically combined with servers of the S-CSCF type so as to constitute servers known as “I/S-CSCF” servers, that act when registering a client device to interrogate a home subscriber server (HSS) in order to be able to select an S-CSCF server that possesses the characteristics that are necessarily required for reaching the level of service to which the user has subscribed (and possibly also characteristics that are optionally required, where appropriate). Each HSS contains a client data database and is thus the equivalent in an IP network of a home location register (HLR) of the kind used in global system for mobile (GSM) networks. Each HSS contains the “profile” of a certain number of client devices of the network, which profile contains their registration states, together with authentication and location data.
After an S-CSCF server has thus been allocated to a user, each user can send a request to subscribe to certain services, which request is valid for the current connection. The general principle is that a client device can subscribe to a particular technical service with the help of an appropriate request (SIP SUBSCRIBE). Thus, when subscribing to the state of a resource, event notifications (SIP NOTIFY) are sent to the client device whenever the state of the resource changes; for example, when the user of a terminal has a voicemail box on the network, the terminal may subscribe to being notified when a message has been left, i.e. it may request to be informed each time a message is recorded in the voicemail box; likewise, the user terminal may request to be notified about its own registration state, and so on.
When a user issues a service request targeting another user (e.g. in the context of making a telephone call), the service request is received by the initial or “originating” S-CSCF server in charge of the user making the request. This S-CSCF server begins by verifying that the requested service is in compliance with the subscription of the requesting user. If so, the S-CSCF server determines whether the target user is known to the IMS network; two situations can then arise:                either the target user is known to the IMS network, in which case the request is transmitted to the I-CSCF server that then routes it to the final or “terminating” S-CSCF server in charge of the target user; or else        the target user is not known to the IMS network, in which case the request is transmitted to a breakout gateway control function (BGCF) server; this BGCF server then consults routing tables and on that basis directs the request:                    either to a media gateway control function (MGCF) server that controls a trunking media gateway function (T-MGF) gateway serving to transfer appropriate media streams to the entry point of a non-IP network to which the target user belongs; or else            to the entry point of an IP network managed by a third party operator.                        